A Surfrider Foundation sponsored Ocean Friendly Garden has been installed in Seattle! On a brisk Saturday morning, over 15 volunteers showed up to help install the rain garden in North Seattle. The preliminary work of ‘plumbing’ the garden’ had been taken care of in the past few weeks, allowing for the volunteers to focus on the more arduous task of digging, grading, laying the rock, planting, and mulching. After a little bit of effort- and fun conversations!- the garden and sunken patio was complete. Now all we need is a good rain storm to help flush it out and to allow for some inevitable fine tuning to maximize performance.

Thanks to all volunteers and the Surfrider Foundation for their support.

First, map out your downspouts. The size of your rain garden will be dependent on the amount of roof area you want to drain.

Determine the volume of water you will be dealing with.

This is easy. Seattle avg. rainfall is 38.6 in/yr or 3.2 ft/ yr. This needs to be multiplied by the roof area your dealing mitigating. In our case it was 763 sf. So, 3.2 ft/ yr x 763 sf = 2442 cf/ yr. That’s a lot. But, to put into into gals, you divide by .14. There are .14 gals in 1 cf. So, 2442 cf/ .14 = 17,443 gals / year! So, in our case, we’re mitigating 35% of this in the rain garden.

.35 x 17,443 gals/ year = 6105 gals/ year of untreated storm water kept out of Piper’s Creek!

Now, you need to size garden to accommodation this volume of water.

Size the rain garden accordingly and your off to the races. Typically, the bottom area of rain gardens need to be about 15% of total runoff area for effective infiltration. Otherwise you will have more of a bio-swale function during heavy rains, etc. The City of Seattle Rainwise program has information regarding rain garden sizing as well. The link below is a good start. If this feels too daunting, it might be a good idea to talk to a professional whom can offer tips and or put together a design plan.